Within trophic level shifts in collagen-carbonate stable carbon isotope spacing are propagated by diet and digestive physiology in large mammal herbivores.

Stable carbon isotope analyses of vertebrate hard tissues such as bones, teeth, and tusks provide information about animal diets in ecological, archeological, and paleontological contexts. There is debate about how carbon isotope compositions of collagen and apatite carbonate differ in terms of their relationship to diet, and to each other. We evaluated relationships between δC and δC among free-ranging southern African mammals to test predictions about the influences of dietary and physiological differences between species. Whereas the slopes of δC-δC relationships among carnivores are ≤1, herbivore δC increases with increasing dietary δC at a slower rate than does δC, resulting in regression slopes >1. This outcome is consistent with predictions that herbivore δC is biased against low protein diet components (C-enriched C grasses in these environments), and δC is C-enriched due to release of C-depleted methane as a by-product of microbial fermentation in the digestive tract. As methane emission is constrained by plant secondary metabolites in browse, the latter effect becomes more pronounced with higher levels of C grass in the diet. Increases in δC are also larger in ruminants than nonruminants. Accordingly, we show that ΔC- spacing is not constant within herbivores, but increases by up to 5 ‰ across species with different diets and physiologies. Such large variation, often assumed to be negligible within trophic levels, clearly cannot be ignored in carbon isotope-based diet reconstructions.